Occurrence of Armillaria rhizomorph populations in the soil beneath indigenous forests in the

The distribution of rhizomorphs of species of Armillaria was determined in indigenous forests at three sites up to 80 km apart in the Rotorua-Bay of Plenty district, New Zealand, by systematically taking cylindrical soil core samples 16 cm diameter by 22 cm deep. There was significant between-site variation in the frequency of occurrence of rhizomorphs (4-19%; p <G.05). At one site with four plots (36 x 28-36 m; up to 1.4 km apart), there was between-plot variation in rhizomorph frequency (13-31%; p < 0.01), mean rhizomorph length per unit area of soil surface (2-9 m/m2; p <0.01), and yield of isolates from samples containing rhizomorphs (41-89%; p <0.01). Distribution of viable rhizomorphs was clustered in two plots (p <0.05). Cultural techniques were used to identify species and intercompatibility groups among isolates made from rhizomorphs and basidiomata. Plots contained 19-93 groups of A. novae-zelandiae (Stevenson) Herink per hectare and 15-58 groups of A. limonea (Stevenson) Boesewinkel per hectare. Dimensions of intercompatibility group clusters varied from less than 4 m to at least 30 ' m across. Five out of eight billets of Beilschmiedia tawa (A. Cunn.) Kirk and Pinus radiata D. Don, protected from soil rhizomorphs by plastic shields, were each colonised by Armillaria species 20-22 months after being partially buried 1-5 m from a dense cluster of sporulating basidiomata of A. novae-zelandiae. This result, together with the high numbers of intercompati-bility group density, suggests that basidiospores may play an important role in the establishment of infection centres in New Zealand forests